survey: add strintmap to count observed refs by prefix #713

Job	Run time
config	3s
fuzz smoke test	35s
win+VS build	0s
linux-musl (alpine)	1m 37s
pedantic (fedora)	1m 51s
linux32 (daald/ubuntu32:xenial)	1m 33s
documentation	3m 35s
linux-asan-ubsan (ubuntu-latest)	2m 5s
linux-gcc (ubuntu-20.04)	3m 10s
linux-gcc-default (ubuntu-latest)	1m 7s
linux-leaks (ubuntu-latest)	45s
linux-reftable (ubuntu-latest)	1m 26s
linux-reftable-leaks (ubuntu-latest)	46s
linux-sha256 (ubuntu-latest)	1m 17s
linux-TEST-vars (ubuntu-20.04)	1m 5s
osx-clang (macos-13)	1m 6s
osx-gcc (macos-13)	1m 27s
osx-reftable (macos-13)	1m 34s
sparse	2m 16s
static-analysis	3m 38s
win build	1m 38s
win+VS test	0s
win test	0s
	32m 34s

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